In general, polystrenes are classified into an atactic, an isotactic and a syndiotactic structure depending on the positioning of benzene rings as side chains. An atactic polystyrene has an irregular arrangement of the benzene rings and an isotactic polystyrene has an arrangement that the benzene rings are positioned at one side of the polymer main chain. On the other hand, an syndiotactic polystyrene has a regularly alternating arrangement of the benzene rings.
Metallocene catalysts are used for preparing the syndiotactic polystyrenes. The metallocene catalysts have a bridged structure of a complex of Group IV transition metals such as Ti, Zr, Hf etc. in the Periodic Table and a ligand having one or two cycloalkane dienyl groups such as cyclopentadienyl group, indenyl groups, fluorenyl group, and derivatives thereof. As the metallocene catalysts have high activities, the catalysts can prepare polymers having better physical properties than the Ziegler-Natta catalysts.
A metallocene catalyst is used with a cocatalyst. A representative example of the cocatalyst is alkyl aluminoxane which is prepared by reacting water with an alkyl aluminum compound. Such catalyst system can prepare polystyrene having a high syndiotactic stereoregularity and a high molecular weight.
European Patent Publication No. 210615 A2 (1987) discloses a syndiotactic polystyrene with stereoregularity which is prepared by using a catalyst of cyclopentadienyl titanium trichloride or alkylated cyclopentadienyl titanium such as pentamethyl cyclopentadienyl titanium trichloride. Such catalysts are known to have preferable catalyst activity, molecular weight, and syndiotactic index.
Japanese Patent Publication Nos. 63-191811 and 3-250007 disclose sulphur bridged metallocene catalysts which have a low yield of manufacture. Also, Japanese Patent Publication Nos. 3-258812, 4-275313 and 5-105712 disclose alkyl bridged metallocene catalysts which have too low a yield of manufacture to commercialize.
U.S. Pat. No. 4,544,762 teaches a process for polymerizing alpha-olefins or styrenes with a high activity and a high stereoregularity using a catalyst system consisting of a transition metal catalyst and a reaction product of alkyl aluminum and metal hydroxide. The catalyst system can prepare polyolefins or polystyrenes with a higher activity and a higher stereoregularity than the Ziegler-Natta catalyst.
Japanese Patent Publication Nos. 62-104818 and 62-187708 disclose metallocene catalysts for preparing polystyrene having a syndiotactic structure. The metallocene catalysts have a transition metal of Group IVB of the Periodic Table and a cyclopentadienyl derivative as ligand. With the catalysts, alkyl aluminoxan is used as cocatalyst, which is a reaction product of an alkyl aluminum with a metal hydroxide.
U.S. Pat. No. 5,026,798 teaches a catalytic process using a Group IVB transition metal component and an aluminoxane component to polymerize alpha-olefins to produce high crystallinity and high molecular weight poly-alpha-olefins.
U.S. patent Ser. No. 08/844109 U.S. Pat. No. 6,010,974 and Ser. No. 08/844,110 abandoned disclose new alkyl-bridged binuclear metallocene catalyst, silyl-bridged binuclear metallocene catalyst, and alkyl-silyl-bridged binuclear metallocene catalyst to polymerize styrenes to produce polystyrene having high stereoregularity, high melting point, and good molecular weight distribution.
A batch process or a continuation process is adopted to prepare polystyrene having a syndiotactic structure. The processes employ a tank-type reactor equipped with agitating blades.
U.S. Pat. No. 5,037,907 discloses a vertical tank-type reactor with a agitating blade. The process is called as solution polymerization because the monomers remain in a liquid phase during polymerization. In theoretical, although this process can prevent the polymer from agglomerating because the liquid media are dispersed on the surface of the polymer particles, this type reactor still causes a problem of a low monomer conversion rate such as 75% below which will result in deteriorating quality of the polymer.
U.S. Pat. No. 5,254,647 discloses a self-cleaning reactor which is capable of continuation process and is a twin screw reactive extruder type. The reactor can prevent agglomeration of polymer through mixing. The wiped surface reactor controls polymerization until about 10 to 20% of the polymer product is obtained in a powder state, and a continuation process is carried out in a powder bed reactor which is a vertical tank type. This reactor can prevent a rapid polymerization. However, it is not economical due to use of two reactors and it has a disadvantage for the wiped surface reactor to limit the process capacity of the system.
U.S. Pat. No. 5,484,862 discloses a liquid phase, powder bed polymerization process for preparing syndiotactic polymers of vinyl aromatic monomers comprising continuously introducing one or more vinyl aromatic monomers and one or more catalyst systems to a horizontally disposed, continuously agitated, cylindrically shaped reactor containing a particulated solid, and continuously removing polymerized product therefrom.
PCT Publication No. 99/10394 discloses a process to produce syndiotactic polymer, which comprises preparing a polymer-containing mixture by polymerizing a first aromatic vinyl monomer with a catalyst at the conversion rate of from 60 to 80% in a first reverse mixing reactor under polymerization condition, and introducing the polymer-containing mixture to a second or more reverse mixing reactors to contact a second aromatic vinyl monomer under polymerization condition.
However, when the conversion rate to polymer is 10% or more, the conventional processes produce large particles having a diameter of 2 mm or more to result in difficult transportation and low drying efficiency. Further, the conventional processes have a shortcoming to form agglomeration of the polymer on the inner surface of the reactor and agitator. The agglomerated material cannot easily be removed because of strong adhesion and causes bad agitation during repolymerization without removing.
Accordingly, the present inventors have developed a process to polymerize styrenes to produce polystyrene having high stereoregularity, high melting point, and good molecular weight distribution, which comprises microfluidizing styrene monomers and catalysts at high pressure to control precisely the polymer particle size to 100 .mu.m or less and not to agglomerate on the inner surface of the reactor.